Method of making light sensitive films



METHOD OF MAKING LIGHT SENSITIVE-FILMS Filed May 2s, 1930 2 sheets-Sheet 1 /J/swsa /jZ v W, y I dwf? vzw?,

Junzs, 1936. L 'T TROLAND' 2,044,864

M-ETHOD OF MAKING LIGHT SENSITIVE FILMS Filed May 23, 1930 2 SheetS-Sheet 2 MEDIUM /NTENII'V HIGH INTENS/TY CURVE CURVE LOW NTENS/TY CURVE v POSITIVE FILM LOW TRANSMISSION MED/UM TRANSMISSION HIGH TRANSMISSION RANGE RANGE RANGE I m h PR/Nr/NG L/GHr s o r B' Vc: y a a r s/ ue GREEN fs/.Low

Patented June 23, 1936 2,044,864 l METHOD OF MAKING LIGHT SENSITIVE FIIMS Leonard T., Troland, Cambridge, Mass., assigner, by mesne assignments, to Technicolor, Inc., New York, N. Y., a corporation of Delaware inplannenV May 23, 1930, semi No. 454,913

15 Claims.

This invention relates to photographic positive emulsions or mms-especially for use in cinematography and cinematographic reproductions (as for example by the imbibition process of dyeing a matrix nlm and printing blank iilms therefrom) in color or in black and white and to a method of preparing and using the same.-

In the art of making photographic images to represent views of objects which present wide ranges of light intensity or contrast (i. e., corresponcng to high-lights, half or middle tones, and shadows) it is important that the appropriate gammas or density gradations shall be attained therein, and shall be consistently obtainable throughout the entire range of intensities and corresponding densities which may occur in any given instance. In photographic images which are formed in the usual light sensitive silver emulsions or 4films such density variations are 30 produced by the relative densities of reduced silver granules and/or depths of penetration of the exposure light into the emulsions, eective to form such metallic silver granules-which correspond in a general way to the logarithm of 25 the relative intensities and/or depthsof penetration of the exposure light, respectively.

In the sensitive silver emulsion ilms as ordi-` narily prepared, however, it is found that such silver imagesare produced primarily by the ultra 30 violet, violet or blue component of the exposure light to which the emulsions usually used are predominantly sensitive and accordingly the emulsions present a relatively narrow or limited range of sensitivity, with respect to the spectral 35 composition of the light to which they are exposed. Moreover, in such images there is a tendency for the gradations of density or gamma in the higher ranges of density in the developed film to be abnormally low, although the interme- 40 diate ranges of densities may be substantially correct in this respect and correspond accurately to the relative intensities ofthe light to which the emulsion has been exposed. Y

In other words, if the density 'of the metallic o silveror depth to which the exposure light has penetrated the emulsion and aiected the formation of granules of silver salt contained thereinrbe plotted against the logarithm of increasing 50 intensities or time of the exposure light, a curve of the. characteristics indicated in Fig. 4 will usually be obtained. As shown by this curve the density increases only gradually in the lower and upper portions of the curve while inthe interme- 55 diate range the density gradation or gamma is more nearly directly proportional to the logarithm of the intensity. Y

It is therefore desirable to extend the range of densities obtainable in the photographic emulsion and to obtain throughout said range density 5 gradation or contrast gradients more accurately to correspond to the range of intensities and relative contrasts presented by the exposure light. It is also desirable under special circumstances, to vary-at will 'and independently the relative con- 10 trasts in regions of different intensities.

Furthermore, if photographic images as heretofore produced in the manner described are formed in physical relief (as by dissolving and removing the undeveloped portions of the gela- 15 tine emulsion) the image which is left presents a relief, the thicknesses of which substantially correspond to the ldensity of the silver image, while the contours or relative thicknesses represent the gradations of density or contrast. In such relief images, however, the same limitations of range of sensitivity to the exposure light and failure of correlation of densities are present as are manifested by the original silver image. Thus, the

higher relief contours (densities) may present lower gradients or slopes than the intermediate contours; `and in' practice such gradient may be reduced by the dissolving operation in these extreme parts of the relief more than in the intermediate portion.

Accordingly, in both typesv of photographic images i. e. whether produced by densities of metallic silver or depth of penetration of the exposure light into the developed emulsion or by Vthe contours in the gelatine relief, the image may be deficient in gamma (i. e., density gradient or relief contour slope) in one or more regions of variation of intensity of the exposure light; and also limited with respect to the range of densities or thicknesses of relief which may be relied 40 upon for accurate contrast reproduction.

Moreover, when films containing such images (and especially gelatine relief images winch are the more commonly employed for the purpose) are wet with a solution of a dye which they selectively adsorb and/or absorb and the dye wet surfaces then applied to a blank lm for the`imbibition transfer of the absorbed dye image to the blank, it is found that the contrast gradients and ranges of contrast obtained in the thus printed dye image are still further reduced. In such cases, the dye absorbed andtransferred by the higher portions ofthe relief image, forms dye images characterized by low gamma or contrast.l Furthermore, such disproportionate absorption imbibition by the blank lm or. (3) permanent retention of a part of the dye by the matrix film. Such disproportionate formation of the dye image, resulting in loss of contrast or gamma, occurs primarily in the areas of high densities or light intensities but may occur anywhere in the range of densities-low, medium or high.

It is therefore desirable and an object of this invention to effect an individual and independent control of the several gammas (i'. e., contrast gradients in the developed emulsion or relative contour gradients in the physical relief images) in each of the regions. of the photographic image corresponding to the low medium and high ranges of light intensity in the exposure light. It is also an object to extend the aggregate range of light intensities which may be reproduced and throughout which such control of the gamma o1 contrast in the photographic image may be effectuated. It is a further object soto control the effective gammas in the photographic image of a matrix film (for imbibition purposes) that the absorption and/or adsorption of a dyesolution and transfer of the same by imbibition to a blank lm shall produce a dye image upon the latter characterized by 'a wider range of contrasts and 'by manifesting the desired gamma or contrast gradient throughout the range of light (color) intensities to be reproduced thereby. It is also an object to vary at will (and independently) the relative contrasts in regions of different intensity. Other objects will appear from the following disclosure.

The. invention includes briey the preparation of a photographically sensitive (silver) gelatine emulsion preferably by treating with one or more sensitizing agents to render the same sensitive to a broader spectral band of light (e. g., corresponding to three spectral zones) and also by dyeing with three (or more) dyes which are severally characterized by being selectively absorptive of light waves in separate spectral zones (preferably distinct but possibly overlapping) within the said sensitized band.

The emulsion thus formed is characterized by a Wider range of sensitivity than the silver emulsions heretofore employed in positive printing. And moreover, by employing a composite light (of Widely varying intensities and comprising light waves in each of the three spectral zones selectively absorbed by the dyes) in exposing the same, the contrast gradients (or relief contours) corresponding to the regions of low medium and .high intensities of the exposure light may be independently controlled by the successive and selective absorption of light of said spectral colors,

respectively.

Preferably the light of shortest Wave length will be sharply absorbed (by the specic absorptive dye therefor) to produce the lower range of densities or reliefs and thereby also to accentuate the gradation of density or contours within this range. The exposure light of intermediate wave length maybe less sharply absorbed (by the dye specifically absorptive thereof) and hence penetrate the emulsion deeper, to produce the range of intermediate densities and/or reliefs. In this range4 the gradations of density or contrast are ordinarily sufficiently well reproduced; but in l this range the use of selectively light-absorptive dye restrains'the corresponding light Waves of the exposure light to the development of the intermediate range of intensities and prevents their further penetration. A portion of the remaining l still less strongly absorbed light (usually of still longer Wave length) is permitted to penetrate more deeply into the gelatine emulsion but, due to the action of the third dye, may not pass entirely through the film and hence is eiectively l5 absorbed therein to form the photographic image in the region of high densities or reliefs, with gradations or contours, (accentuated still more than in the regions of low densities or reliefs) and hence provides the required contrast in this range of light intensity. 4

The specific absorption of light effected in each spectral zone of the exposure light may be in such proportion to the sensitivity of the emulsion With respect to light waves of that zone as to reduce the depth of penetration or thickness of relief produced by the penetration of the exposure light, to the absolute (and relative) degree required. Accordingly the density gradient or relief contour produced, corresponding to each zone of light intensity in the exposure, may be controlled by controlling the ratio of the absorptivity of the dye to specific sensitivity of the lm to the absorbed light, as may be desired.

. For example, a silver emulsion may be rendered sensitive to blue-green, green and yellow light (in addition to the normal sensitivity to blue light), thus presenting a relatively Wide zone of spectral sensitivity (or to three separate spectral zones) if desired). It may also be treated with dyes which are selectively absorptive of blue, blue-green, green and yellow light, respectively. The dyes may be employed, severally, in appropriate amounts or concentrations in relation to the sensitivity of the emulsion to light of the several spectral wave lengths, to correspond to and control the formation of the photographic image produced by the low, medium and high intensities of the exposure light. Thus a dye strongly absorptive of blue light may be employed in a film to reduce the density of an image (or thickness of its relief) to be produced thereon by the blue zone of the exposure light to represent the low intensities of the exposure light, and the degree of reduction will be controlled by the amount of dye used and varied within such range by the relative intensities of the (blue component of the) exposure light employed. Thus, the gamma or relative degree of density or thickness of the-relief image produced may be controlled by the relationship of the, specic absorption characteristics of the dye to the sensitivity characteristic of the emulsion in this spectral range, and determined by the quality, time and intensity of the blue component of the exposure light to which it is subjected.

Likewise, the blue-green and green components of the exposure light, to which the emulsion has beenY sensitized, may be selectively absorbed by the second -dye and the depth of penetration or thickness of -the relief image formed thereby may be reduced. But it may, for example, be reduced less than the depth of penetration of the image or thickness of reliefproducedby the blue light,- thereby to produce a region of deeper penetration of the emulsion and a higher relief in the resulting image, consequent upon this component and corresponding to the intermediate range of intensities of the exposure light. The gamma or gradation of relief produced in this zone oi.' the exposure light will likewisev be controlled by the relationship of the sensitizing and absorptive values of the reagents, in conjunction with the variations in intensities of the exposure light.

The third (e. g. greenish-yellow) component of the exposure light (to which the emulsion has beensensitized) may also be selectively absorbed by the presence of the third dye and the depths of penetration (or thicknesses of the relief image) formed thereby likewise somewhat reduced. But the degree of reduction of penetration in this zone may be relatively slight so as to permit deep penetrations of the emulsion by thisA component of the exposure light (limited only by the thickness of the film) thus to produce the high reliefs in the resulting photographic image and sharper contrast or contour gradients corresponding to the higher range of intensities of the exposure light.

` In the above example, the positive film emulsion f (shown in section) according to the invention is throughout its thickness sensitive to blue, green and yellow light (that is, isochromatic in contradistinction to the positive emulsion normally used) and, likewise throughout its thickness, selectivelyiabsorptive of light in three zones vin the blue, green and yellow regions of the spectrum. This absorption is, according to the invention, stepped in such a manner that substantially all blue exposure light is absorbed within depth b next to the light entrant side of the emulsion.

The absorption in the green zone is less strong, so that green printing light penetrates depth y. 'Ihe absorption of yellowprinting light is still less intensive. so that the latter penetrates depth y, that is, substantially the entire emulsion.

It will s now be evident what happens when emulsion f is exposed to a record to be reproduced, for example a negative n represented by developed silver wedge w. The printing light contains at `least the blue, green and yellow ranges indicated by rays B, G, and Y, but may be white, for` obvious reasons. The highly absorbed blue printing light is exhausted within region b, and the exposure effected by it can be indicated by curve L representing the low intensities or high light portions of the positive, within sector l of the wedge. Similarly, the green light aiects the medium intensity record M corresponding to region 'm and the yellow light the high intensity or shadow record H, corresponding to region h of the negative.' Hence, the shape of the density exposure curve (or the contrast characteristic) of the total record can be controlled and adjusted by substantially independently controlling the exposure within each of three depths b, g, y. The contrast control within each independent contrast zone or band as well as the depth of penetration for eachlband can be controlled by adjusting sensitivity, absorption and printing light'v intensity for that band, zone or depthof penetration.

In the embodiment described, the gelatine portions abovevlines L, M, H would be dissolved or etched off, so that the resulting Across section of the relief'reproduction of -wedge w indicates diof the invention will be described with reference posi-te absorption values of the dyes employed and also a corresponding graph (F) showing the composite spectral sensitivity of the iilm;

Fig. 3 represents the cumulative penetration or reliefs of an image resulting from exposure `of the sensitized and dyed lm of Fig. 2 to increasing intensities, of exposure light and the gammas obtained in the low medium-and high intensity regions, respectively, in accordance with this invention;

Fig. 4 represents the usual graph or relief of an image resulting from exposure of an ordinary sensitive gelatine emulsion to increasing intensities oi' exposure light; and

Fig. 5 is a diagram explaining the new method and including a section through an emulsion forming an embodiment of the invention.

In Fig. 1, the graphs indicate the spectral absorption value of (A) tartrazine, (B) rose bengel (C) acid magenta (D) erythrosine components of the dye solution employed (given below) plotted in terms of thefactor:

lo Transmitted light gw Incident light as ordinates and spectral wave lengths as Vabscissae.

In Fig. 2 the graph'E represents, in the same manner, the composite effects of the several dyes, While graph F indicates the spectral sensitivity ofthe lm as sensitized primarily by silver halide in the region F', and by rose bengaland/or erythrosine in the region F. The latter dye or sensitizer develops an excessive sensitivity to light in the zone-between 550 and 620. Consequently, by its addition to the emulsion there is an `overcorrection and extension of sensitivity in these spectral zones. This may be partially oiset or neutralized by the use of a filter (such as cobalt glass) but it is preferred, in accordance with the invention, to more accurately adjust the same by the addition of supplementary absorptive dyes. To this end it is found that rose bengal selectively absorbs light in the region 5304380; and that acid magenta selectively absorbs light in the region 580-620. These two dyes, therefore, are especially effective to absorb light in the zones which are thus rendered sensitive by the erythrosine, and hence to adjust the relief produced in the exposed lm by light in tliese spectral zones independently and additively, and thus to control the height of the relief (or density) of the silver negative and the gamma of the positive produced therefrom, to accord with the density or relief, andgamma produced in the blue zone, for example. It is to be observed that in the spectral zone F' (toy which the emulsion is sensitive without erythrosine) the tartrazine is strongly absorptive. This results in confining the penetration of this component of the exposure light tolow values and particularly to the region of lower relief contours.- The relief contours in this region will,

however,' be directly related to the intensity of this component of the 'exposure -light and hence possess accurate proportionality and take up and transfer proportional amounts or concentrations of dye to a printed blank, by imbibition, and especially to reproduce sharper contrasts in the lower range of intensities.

In thev spectral zone made sensitive by the erythrosine (F" the sensitivity may be unnecessarily great in comparison to the sensitivity of the silver salt as limited by tartrazine. Of this zone, the spectral component of the exposure light in the lower portion may be sharply abssrbed by rose bengal, while that in the higher portion may be (less sharply) absorbed by acid magenta and allowed to penetrate still more deeply into the lm. In both portions of this sensitized zone the contrast gradation or gamma (relative depths of penetration or thickness of relief) will be proportionate to the intensities of the respective components of the exposure light,

l and controlled by the specific ratio of sensitiviiy" Grams Gelatine 100 Tartrazine 10 Rose bengal, 0.750 Acid magenta 0.125

A transparent lm is then coated with the emulsion and allowed to dry, when it is ready for use in the customary manner. The lm mayl then be exposed, for example, through a suitable' negative to light having a, spectral range corresponding to the aggregate spectral range of absorption of the composite dye-or to the spectral range of sensitivity of the film, and then converted into a relief image, as for example in accordance with my Patent 1,535,700. Likewise the exposed stock may be developed in any of the usual non-tanning development solutions, and, if desired, xed in the usual way with hypo.

The resulting silver image may then be converted into a gelatin relief image,'as by bleaching in a solution of ammonium dichromate and potassium bromide, sulphuric acid and water, or

in a solution of chromicacid, sodium chlorideA and'w-ater, or potassium dichromatev and hydrochloric acid, or potassium ferricyanide, dichromate, copper sulfate, and hydrochloric acid.

These solutions cause hardening of the gelatin in those parts of the lm which contain silver, and permit the removal of the unhardened gelatin with hot water. Subsequent treatment with hypo followed by Washing and drying, completes the operation of producing a gelatin relief image.

In the resulting relief images, in whatever suitable way adapted for developing them, the thicknesses and contours produced by the normal (silver sensitive) spectral region of the exposure light are low due to the absorption eii'ect of the tartrazine, the contours of the middle portion (lower erythrosine sensitized) region are intermediate and controlled by the absorption effect of the rose bengal, while the upper (erythrosine sensitized) region which, per se,might be too high, is controlled by the relative absorption effeet of the acid magenta.

The relief image thus obtained is eminently suitable for use in imbibition printing, and serves to produce an accurate printed image, in which the proper contrasts are reproduced throughout the entire range of intensities of the exposure light,-and the range of such contrasts is much greater than has heretofore been obtainable by procedures or compositions of the prior art.

vMoreover, any specic variations which may be encountered (due to the dye employed, the film blank to be printed, etc.) in the low, medium or high region of light intensities may be severally corrected, without affecting the other and correctly reproduced regions.

I claim:

1. In the art of reproducing photographic records, the method of controlling the exposure density relation of lin prints which comprises sensitizing an emulsion forlight inclu-ding a Wave length range of at least approximately millimicrons, making said emulsion absorptive of light of at least three substantial wave length zones substantially covering said range, the degrees of light absorption for the respective zones being stepped so that exposure light of said range penetrates to three substantial depths corresponding to said z ones stepped throughout the emulsion, printingl on said emulsion with light including said range and adjusting the exposure density relation of each zone through absorption of the emulsion and exposure light intensity, thereby controlling the exposures of said .zones substantially independently and thus adjusting the shape of the-total density exposure relation.

2. In the art -of reproducing photographic recl ords the method of controlling the vexposure density relation of film prints which comprises rendering an emulsion throughout its depth sensitive to, and absorptive of, three substantial wave length zones representing together a spectral range of at least approximately 120 millimicrons, said `sensitivity and absorption being stepped for said three' zones so that exposure light of said range penetrates to three vsubstantial depths corresponding to said zones stepped throughout the emulsion, printing on said emulsion with' light including said range and adjusting the exposure of each zone with sensitivity and absorption of the emulsion and exposure light intensity, thereby substantially independently controlling vthe exposures of said zones and thus adjusting the shape of the totaldensity exposure relation. 3. In the art of reproducing photographic records, the vmethod of controllingv the exposure density relation of film prints which comprises rendering any emulsion throughout its -depth sensitive to, and absorptive of, three distinct substantial wave length zones representing together a spectral range of yat least approximately 120 millimicrons, said sensitivity and absorptionl being stepped for said three zones so that expo-sure light of said range penetrates to three substantial depths corresponding to said zones throughout the emulsion, printing on said emulsion with light of said range and independently adjusting the exposure-density relation of each zone by adjusting the` intensity of-its exposure light, thereby substantially independently controlling the exposures -of said zones and thus adjusting the shape of the total density. exposure relation.

'4. In the art of reproducing photographic rec- 75 ords, the method of controlling the exposure density relation of film prints which comprises sensitizing an emulsion throughout its depth for light including a Wave length range representing l at least approximately 120 millimicrons, making said emulsion throughout its depth absorptive of light of at least three substantial wave length zones substantially covering said range by incorporating therein three dyes acting as ilters absorbing light of wave lengths of said zones, the degrees of absorption for the respective zones' being stepped so that exposure light of wave lengths of said range penetrates to three s ubstantial depths corresponding to said zones stepped throughout the emulsion, printing on said emulsion with light including said range, and controlling the conguration of the density exposure relation for each zone and depth substantially independently by adjusting sensitivity and absorption of the emuls 'in and exposure light intensity, thereby controllingthe shape of the total density exposure relation.

5. Reproduction lm comprising an emulsion sensitized throughout for light including a wave length range of at least approximately 120 millimicrens, and at least three dyes acting as iilters incorporated throughout the emulsion, saiddyes absorbing light of at least three substantial wave length zones substantially covering said range, the light absorption degree of said dyes being stepped for penetration of light of the wave lengths of said zones to three substantial depths corresponding to saidzones stepped throughout the emulsion.

6.,Reproduction iilm comprising an emulsion sensitized throughout for approximately the blue, green and yellow wave lengths, and dyes acting as lters incorporated throughout the emulsion, said dyes absorbing blue, green and .yellow light, the respective light absorption degree of said dyes being stepped for penetration of blue, green and yellow light, respectively, to three substantial depths corresponding to said zones stepped throughout the emulsion. i

'7. The method of making reproduction lm which comprises sensitizing the lm emulsion for light including a wave length range of at least approximately 120 millimicrons, and treating the same with at least three dyes selectively absorptive of light of at least three substantial wave length `zones substantially covering said range, the degrees of light absorption of said dyes being stepped so that exposure light of saidrange penetrates to three substantial depths corresponding to said zones stepped throughout the emulsion.

8. The method of making reproduction lm which comprises rendering its emulsion throughout its depth sensitive for a wave length range of at least approximately 120 millimicrons, and incorporating therein, substantially throughout its depth, at least three dyes acting as lters, said dyes absorbing light of at least three substantial VWave length zones substantially covering said range, the light absorption degree of said dyes be-l ing stepped for penetration of light of the wave lengths of said zones to three substantial depths corresponding to said zones stepped throughout the emulsion.

9. The method of making reproduction film, which comprises the steps of rendering the lm emulsion sensitive to all-of the spectral zones of White exposure light and treating the same with three dyes, said dyes being selectively absorptive of light in the ultra violet to blue zone, in the blue green to greenish yellow zone, and in the yellowred zone, respectively, of the white exposure light, said selective absorption being stepped for y the A penetration of the exposure light of wave lengths of the respectivezones to three substantial depthsI .1(i. The method of making light sensitive film,

which comprises the steps of rendering the film emulsion sensitive to all of the spectral zones of white exposure light, and, treating the same with three dyes, one being selectively and strongly absorptive of light in the ultra violet to blue zone, the second being selectively and moderately absorptive of light in the blue green to greenish yellow zone, and the third being selectively and slightly absorptive of light in the yellow to red zone, of the white exposure light.

11. The method-of making matrix i'ilm suitable for imbibition printing, which comprises the steps of rendering the same sensitive to all of the spectral zones of white exposure light and treating the same with three dyes, said dyes being selectively absorptive of light in the ultra violet to blue zone, in the blue green to greenish yellow zone, and in the yellow-red zone, respectively, of the white exposure light, said selective absorption being stepped for the penetration of the exposure light of wave lengths of said zones to three substantial depths corresponding to said zones throughout the emulsion, subjecting the emulsion to exposure light of varying intensities containing the wave lengths of said zones, 'developing the exposed lm, and removing the undeveloped portions of the emulsion.

12. The method of making matrix iilm suitable for imbibition printing, which comprises the steps of rendering the iilm emulsion sensitive to all ofthe spectral zones of White exposure light, and treating the same with three dyes, one being selectively andstrongly absorptive of light in the ultra violet to blue zone, the second being selectively and moderately absorptive of light in the blue green to greenish yellow' zone, and the third being selectively and slightly absorptive of light in the yellow to red zone, of the white exposure light, subjecting the emulsion to white exposure light of varying intensities, developing the exposed lm and removing the undeveloped portions of the emulsion.

13. Method of making photographic emulsion for the development of a Wide range of contrasts V light of shorter wave length zone, to produce low contrast gradients in the normally sensitive region, rendering the emulsion selectively absorptive with respect to light of intermediate wave length to produce an intermediate region of -densities or contrast gradient contours corresponding to the lower erythrosine sensitized zone, and rendering the emulsion selectively absorptive with respect to light of longer wave length, to produce the higher contrast gradient in the region of greater densities corresponding to the upper erythrosine sensitized zone.

14. Method of making photographie emulsion for the development of a wide range of contrasts therein comprising sensitizlng the same with a silver salt and with erythrosine, andrendering the same 'selectively absorptive with respect to the zone of light of short wave length, by the addition of tartr-azine to produce low densities or contours in the normally sensitive region, renrespect to light of intermediate wave length to produce an intermediate density or contours region corresponding to the lower erythrosine sensitized zone, by the addition of rose bengal, and rendering the emulsion selectively absorptive with respect to light of longer wave length, to produce the higher density or contours region, corresponding to the upper erythrosine sensitized zone by the addition of acid magenta.

15. Method of. making photographic emulsion for the development of a wide range of contrasts therein comprising sensitizing the same with a silver salt and with rose bengal, and rendering the same selectively absorptive with respect to dering the emulsion selectively absorptive with the zone of light of short wave length, by the addition of tartrazine to produce low densities or contours in the normally sensitive region, rendering the emulsion selectively absorptive with respect to light of intermediate wave length to produce an intermediate density or contours region corresponding to the lower rose bengal sensitized zone, by the addition of rose bengal, and rendering the emulsion selectively absorptive With respect to light of longer Wave length, to produce the higher density or` contours region, corre sponding to the upper sensitized zone by the addition of acid magenta.

LEONARD T. TROLAND. 

